GTD setup recommendations are at http://www.davidberman.com/software/timetohelp/GTD.htm

Data can be manipulated via an API

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Short details

Can run entirely from any USB flash memory drive, so you can work your schedule on any available Windows computer (without re-installing). World time zones/daylight saving, Skype dial, free SMS alarms, automatic work/life balance and life goals prioritization, likely the most detailed task recurring properties out there, automatic task splitting, full international support, input/search in 17 languages.

Asana connects your tasks and projects to anyone you want to work with. Use of workspaces to organize all the teams and collaborate with other people while keeping everything separate. All history and discussions for each task are kept together and updates you of the changes to the tasks and projects.